Jolly Family. E-book. Formato PDF Nikol Nosov   -  Forgotten Books, 2017  - 

We set to work to (figure out how many more eggs there would be if all the hens laid eggs instead of hatching chickens. It takes twenty one days for a brood-hen to hatch chickens, and if you count the time she spends looking after them when they're hatched you find that it takes about three months before she starts laying again.